I think what he means is that about the current one, and this one. I would focus on the other arenas first, and then try to do this one again. Also by detail what he means is that you could have made a few well, decorations I suppose you would call it. Make some houses on the mountain side and layer the bottom of the mountains and things like that. The actual fighting arena needs to be flat, with no interruption areas like water and big hills in the middle, but things like what I have just said could be added outside of the fighting area
If im honest, I should have put that in the original post, but that's my advice for making a 1v1 arena.